This paper describes the results of the design effort for a three-mirror anastigmatic system (TMAS) and its stray light analyzing. The system’s focal length is 4000mm and the relative aperture is f/10, it’s total length is nearly f’/3~f’/4 and field of view (FOV) is 30 × 10. The final optical system is a three-mirror unobscured telescope. It is telecentric and achieves diffraction-limited imagery at visible wavelengths. After analyzing the stray light used the Light Tool software, finally We can draw two conclusions: 1) design of unobscured optical system is still best approached using fundamental optical design principle; 2) time spent in careful analyzing the stray light and a higher probability of success for the assembled system.